The assumed final in vivo experiment to determine whether the cotton rat would be a useful small animal model for the study of HIV-1 infections addressed the question: are newborn animals (3 days after birth) more susceptible than adult cotton rats? All of the newborns became infected but none developed signs of disease. At the request of the Office of AIDS Research, however, a study was initiated to determine whether it is possible to make the cotton rat susceptible to a pathogenic infection. For this purpose cotton rat cells were transformed with type 5 adenovirus E1 DNA to produce continuous cell lines. These cell lines are now to be transformed with genes for human CD4 receptor, and human coreceptors to determine whether these cells will then support productive HIV-1 replication.